package org.oaix.jut;

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

import java.util.concurrent.TimeUnit;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import java.util.concurrent.locks.AbstractOwnableSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.LockSupport;

import sun.misc.Unsafe;

/**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues.  This class is designed to
 * be a useful basis for most kinds of synchronizers that rely on a
 * single atomic {@code int} value to represent state. Subclasses
 * must define the protected methods that change this state, and which
 * define what that state means in terms of this object being acquired
 * or released.  Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. Subclasses can maintain
 * other state fields, but only the atomically updated {@code int}
 * value manipulated using methods {@link #getState}, {@link
 * #setState} and {@link #compareAndSetState} is tracked with respect
 * to synchronization.
 *
 * <p>Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class.  Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface.  Instead it defines methods such as
 * {@link #acquireInterruptibly} that can be invoked as
 * appropriate by concrete locks and related synchronizers to
 * implement their public methods.
 *
 * <p>This class supports either or both a default <em>exclusive</em>
 * mode and a <em>shared</em> mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. This class
 * does not &quot;understand&quot; these differences except in the
 * mechanical sense that when a shared mode acquire succeeds, the next
 * waiting thread (if one exists) must also determine whether it can
 * acquire as well. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * <p>This class defines a nested {@link XaAbstractQueuedSynchronizer.ConditionObject} class that
 * can be used as a {@link Condition} implementation by subclasses
 * supporting exclusive mode for which method {@link
 * #isHeldExclusively} reports whether synchronization is exclusively
 * held with respect to the current thread, method {@link #release}
 * invoked with the current {@link #getState} value fully releases
 * this object, and {@link #acquire}, given this saved state value,
 * eventually restores this object to its previous acquired state.  No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it.  The
 * behavior of {@link XaAbstractQueuedSynchronizer.ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * <p>This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * <p>Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * <p>To use this class as the basis of a synchronizer, redefine the
 * following methods, as applicable, by inspecting and/or modifying
 * the synchronization state using {@link #getState}, {@link
 * #setState} and/or {@link #compareAndSetState}:
 *
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}.  Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the <em>only</em> supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * <p>You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer.  You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * <p>Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies.  The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 *     while (!tryAcquire(arg)) {
 *        <em>enqueue thread if it is not already queued</em>;
 *        <em>possibly block current thread</em>;
 *     }
 *
 * Release:
 *     if (tryRelease(arg))
 *        <em>unblock the first queued thread</em>;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * <p id="barging">Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 * others that are blocked and queued.  However, you can, if desired,
 * define {@code tryAcquire} and/or {@code tryAcquireShared} to
 * disable barging by internally invoking one or more of the inspection
 * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 * In particular, most fair synchronizers can define {@code tryAcquire}
 * to return {@code false} if {@link #hasQueuedPredecessors} (a method
 * specifically designed to be used by fair synchronizers) returns
 * {@code true}.  Other variations are possible.
 *
 * <p>Throughput and scalability are generally highest for the
 * default barging (also known as <em>greedy</em>,
 * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 * While this is not guaranteed to be fair or starvation-free, earlier
 * queued threads are allowed to recontend before later queued
 * threads, and each recontention has an unbiased chance to succeed
 * against incoming threads.  Also, while acquires do not
 * &quot;spin&quot; in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking.  This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. If so desired, you can
 * augment this by preceding calls to acquire methods with
 * "fast-path" checks, possibly prechecking {@link #hasContended}
 * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 * is likely not to be contended.
 *
 * <p>This class provides an efficient and scalable basis for
 * synchronization in part by specializing its range of use to
 * synchronizers that can rely on {@code int} state, acquire, and
 * release parameters, and an internal FIFO wait queue. When this does
 * not suffice, you can build synchronizers from a lower level using
 * {@link java.util.concurrent.atomic atomic} classes, your own custom
 * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 * support.
 *
 * <h3>Usage Examples</h3>
 *
 * <p>Here is a non-reentrant mutual exclusion lock class that uses
 * the value zero to represent the unlocked state, and one to
 * represent the locked state. While a non-reentrant lock
 * does not strictly require recording of the current owner
 * thread, this class does so anyway to make usage easier to monitor.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 *  <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 *   // Our internal helper class
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     // Reports whether in locked state
 *     protected boolean isHeldExclusively() {
 *       return getState() == 1;
 *     }
 *
 *     // Acquires the lock if state is zero
 *     public boolean tryAcquire(int acquires) {
 *       assert acquires == 1; // Otherwise unused
 *       if (compareAndSetState(0, 1)) {
 *         setExclusiveOwnerThread(Thread.currentThread());
 *         return true;
 *       }
 *       return false;
 *     }
 *
 *     // Releases the lock by setting state to zero
 *     protected boolean tryRelease(int releases) {
 *       assert releases == 1; // Otherwise unused
 *       if (getState() == 0) throw new IllegalMonitorStateException();
 *       setExclusiveOwnerThread(null);
 *       setState(0);
 *       return true;
 *     }
 *
 *     // Provides a Condition
 *     Condition newCondition() { return new ConditionObject(); }
 *
 *     // Deserializes properly
 *     private void readObject(ObjectInputStream s)
 *         throws IOException, ClassNotFoundException {
 *       s.defaultReadObject();
 *       setState(0); // reset to unlocked state
 *     }
 *   }
 *
 *   // The sync object does all the hard work. We just forward to it.
 *   private final Sync sync = new Sync();
 *
 *   public void lock()                { sync.acquire(1); }
 *   public boolean tryLock()          { return sync.tryAcquire(1); }
 *   public void unlock()              { sync.release(1); }
 *   public Condition newCondition()   { return sync.newCondition(); }
 *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 *   public void lockInterruptibly() throws InterruptedException {
 *     sync.acquireInterruptibly(1);
 *   }
 *   public boolean tryLock(long timeout, TimeUnit unit)
 *       throws InterruptedException {
 *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 *   }
 * }}</pre>
 *
 * <p>Here is a latch class that is like a
 * {@link java.util.concurrent.CountDownLatch CountDownLatch}
 * except that it only requires a single {@code signal} to
 * fire. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 *  <pre> {@code
 * class BooleanLatch {
 *
 *   private static class Sync extends AbstractQueuedSynchronizer {
 *     boolean isSignalled() { return getState() != 0; }
 *
 *     protected int tryAcquireShared(int ignore) {
 *       return isSignalled() ? 1 : -1;
 *     }
 *
 *     protected boolean tryReleaseShared(int ignore) {
 *       setState(1);
 *       return true;
 *     }
 *   }
 *
 *   private final Sync sync = new Sync();
 *   public boolean isSignalled() { return sync.isSignalled(); }
 *   public void signal()         { sync.releaseShared(1); }
 *   public void await() throws InterruptedException {
 *     sync.acquireSharedInterruptibly(1);
 *   }
 * }}</pre>
 *
 * @since 1.5
 * @author Doug Lea
 */
public abstract class XaAbstractQueuedSynchronizer
		extends AbstractOwnableSynchronizer
		implements java.io.Serializable {

	private static final long serialVersionUID = 7373984972572414691L;

	/**
	 * Creates a new {@code AbstractQueuedSynchronizer} instance
	 * with initial synchronization state of zero.
	 */
	protected XaAbstractQueuedSynchronizer() { }

	/**
	 * Wait queue node class.
	 *
	 * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
	 * Hagersten) lock queue. CLH locks are normally used for
	 * spinlocks.  We instead use them for blocking synchronizers, but
	 * use the same basic tactic of holding some of the control
	 * information about a thread in the predecessor of its node.  A
	 * "status" field in each node keeps track of whether a thread
	 * should block.  A node is signalled when its predecessor
	 * releases.  Each node of the queue otherwise serves as a
	 * specific-notification-style monitor holding a single waiting
	 * thread. The status field does NOT control whether threads are
	 * granted locks etc though.  A thread may try to acquire if it is
	 * first in the queue. But being first does not guarantee success;
	 * it only gives the right to contend.  So the currently released
	 * contender thread may need to rewait.
	 *
	 * <p>To enqueue into a CLH lock, you atomically splice it in as new
	 * tail. To dequeue, you just set the head field.
	 * <pre>
	 *      +------+  prev +-----+       +-----+
	 * head |      | <---- |     | <---- |     |  tail
	 *      +------+       +-----+       +-----+
	 * </pre>
	 *
	 * <p>Insertion into a CLH queue requires only a single atomic
	 * operation on "tail", so there is a simple atomic point of
	 * demarcation from unqueued to queued. Similarly, dequeuing
	 * involves only updating the "head". However, it takes a bit
	 * more work for nodes to determine who their successors are,
	 * in part to deal with possible cancellation due to timeouts
	 * and interrupts.
	 *
	 * <p>The "prev" links (not used in original CLH locks), are mainly
	 * needed to handle cancellation. If a node is cancelled, its
	 * successor is (normally) relinked to a non-cancelled
	 * predecessor. For explanation of similar mechanics in the case
	 * of spin locks, see the papers by Scott and Scherer at
	 * http://www.cs.rochester.edu/u/scott/synchronization/
	 *
	 * <p>We also use "next" links to implement blocking mechanics.
	 * The thread id for each node is kept in its own node, so a
	 * predecessor signals the next node to wake up by traversing
	 * next link to determine which thread it is.  Determination of
	 * successor must avoid races with newly queued nodes to set
	 * the "next" fields of their predecessors.  This is solved
	 * when necessary by checking backwards from the atomically
	 * updated "tail" when a node's successor appears to be null.
	 * (Or, said differently, the next-links are an optimization
	 * so that we don't usually need a backward scan.)
	 *
	 * <p>Cancellation introduces some conservatism to the basic
	 * algorithms.  Since we must poll for cancellation of other
	 * nodes, we can miss noticing whether a cancelled node is
	 * ahead or behind us. This is dealt with by always unparking
	 * successors upon cancellation, allowing them to stabilize on
	 * a new predecessor, unless we can identify an uncancelled
	 * predecessor who will carry this responsibility.
	 *
	 * <p>CLH queues need a dummy header node to get started. But
	 * we don't create them on construction, because it would be wasted
	 * effort if there is never contention. Instead, the node
	 * is constructed and head and tail pointers are set upon first
	 * contention.
	 *
	 * <p>Threads waiting on Conditions use the same nodes, but
	 * use an additional link. Conditions only need to link nodes
	 * in simple (non-concurrent) linked queues because they are
	 * only accessed when exclusively held.  Upon await, a node is
	 * inserted into a condition queue.  Upon signal, the node is
	 * transferred to the main queue.  A special value of status
	 * field is used to mark which queue a node is on.
	 *
	 * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
	 * Scherer and Michael Scott, along with members of JSR-166
	 * expert group, for helpful ideas, discussions, and critiques
	 * on the design of this class.
	 */
	static final class Node {
		/** Marker to indicate a node is waiting in shared mode */
		static final XaAbstractQueuedSynchronizer.Node SHARED = new XaAbstractQueuedSynchronizer.Node();
		/** Marker to indicate a node is waiting in exclusive mode */
		static final XaAbstractQueuedSynchronizer.Node EXCLUSIVE = null;

		/** waitStatus value to indicate thread has cancelled */
		static final int CANCELLED =  1;
		/** waitStatus value to indicate successor's thread needs unparking */
		static final int SIGNAL    = -1;
		/** waitStatus value to indicate thread is waiting on condition */
		static final int CONDITION = -2;
		/**
		 * waitStatus value to indicate the next acquireShared should
		 * unconditionally propagate
		 */
		static final int PROPAGATE = -3;

		/**
		 * Status field, taking on only the values:
		 *   SIGNAL:     The successor of this node is (or will soon be)
		 *               blocked (via park), so the current node must
		 *               unpark its successor when it releases or
		 *               cancels. To avoid races, acquire methods must
		 *               first indicate they need a signal,
		 *               then retry the atomic acquire, and then,
		 *               on failure, block.
		 *   CANCELLED:  This node is cancelled due to timeout or interrupt.
		 *               Nodes never leave this state. In particular,
		 *               a thread with cancelled node never again blocks.
		 *   CONDITION:  This node is currently on a condition queue.
		 *               It will not be used as a sync queue node
		 *               until transferred, at which time the status
		 *               will be set to 0. (Use of this value here has
		 *               nothing to do with the other uses of the
		 *               field, but simplifies mechanics.)
		 *   PROPAGATE:  A releaseShared should be propagated to other
		 *               nodes. This is set (for head node only) in
		 *               doReleaseShared to ensure propagation
		 *               continues, even if other operations have
		 *               since intervened.
		 *   0:          None of the above
		 *
		 * The values are arranged numerically to simplify use.
		 * Non-negative values mean that a node doesn't need to
		 * signal. So, most code doesn't need to check for particular
		 * values, just for sign.
		 *
		 * The field is initialized to 0 for normal sync nodes, and
		 * CONDITION for condition nodes.  It is modified using CAS
		 * (or when possible, unconditional volatile writes).
		 */
		volatile int waitStatus;

		/**
		 * Link to predecessor node that current node/thread relies on
		 * for checking waitStatus. Assigned during enqueuing, and nulled
		 * out (for sake of GC) only upon dequeuing.  Also, upon
		 * cancellation of a predecessor, we short-circuit while
		 * finding a non-cancelled one, which will always exist
		 * because the head node is never cancelled: A node becomes
		 * head only as a result of successful acquire. A
		 * cancelled thread never succeeds in acquiring, and a thread only
		 * cancels itself, not any other node.
		 */
		volatile XaAbstractQueuedSynchronizer.Node prev;

		/**
		 * Link to the successor node that the current node/thread
		 * unparks upon release. Assigned during enqueuing, adjusted
		 * when bypassing cancelled predecessors, and nulled out (for
		 * sake of GC) when dequeued.  The enq operation does not
		 * assign next field of a predecessor until after attachment,
		 * so seeing a null next field does not necessarily mean that
		 * node is at end of queue. However, if a next field appears
		 * to be null, we can scan prev's from the tail to
		 * double-check.  The next field of cancelled nodes is set to
		 * point to the node itself instead of null, to make life
		 * easier for isOnSyncQueue.
		 */
		volatile XaAbstractQueuedSynchronizer.Node next;

		/**
		 * The thread that enqueued this node.  Initialized on
		 * construction and nulled out after use.
		 */
		volatile Thread thread;

		/**
		 * Link to next node waiting on condition, or the special
		 * value SHARED.  Because condition queues are accessed only
		 * when holding in exclusive mode, we just need a simple
		 * linked queue to hold nodes while they are waiting on
		 * conditions. They are then transferred to the queue to
		 * re-acquire. And because conditions can only be exclusive,
		 * we save a field by using special value to indicate shared
		 * mode.
		 */
		XaAbstractQueuedSynchronizer.Node nextWaiter;

		/**
		 * Returns true if node is waiting in shared mode.
		 */
		final boolean isShared() {
			return nextWaiter == SHARED;
		}

		/**
		 * Returns previous node, or throws NullPointerException if null.
		 * Use when predecessor cannot be null.  The null check could
		 * be elided, but is present to help the VM.
		 *
		 * @return the predecessor of this node
		 */
		final XaAbstractQueuedSynchronizer.Node predecessor() throws NullPointerException {
			XaAbstractQueuedSynchronizer.Node p = prev;
			if (p == null)
				throw new NullPointerException();
			else
				return p;
		}

		Node() {    // Used to establish initial head or SHARED marker
		}

		Node(Thread thread, XaAbstractQueuedSynchronizer.Node mode) {     // Used by addWaiter
			this.nextWaiter = mode;
			this.thread = thread;
		}

		Node(Thread thread, int waitStatus) { // Used by Condition
			this.waitStatus = waitStatus;
			this.thread = thread;
		}
	}

	/**
	 * Head of the wait queue, lazily initialized.  Except for
	 * initialization, it is modified only via method setHead.  Note:
	 * If head exists, its waitStatus is guaranteed not to be
	 * CANCELLED.
	 */
	private transient volatile XaAbstractQueuedSynchronizer.Node head;

	/**
	 * Tail of the wait queue, lazily initialized.  Modified only via
	 * method enq to add new wait node.
	 */
	private transient volatile XaAbstractQueuedSynchronizer.Node tail;

	/**
	 * The synchronization state.
	 */
	private volatile int state;

	/**
	 * Returns the current value of synchronization state.
	 * This operation has memory semantics of a {@code volatile} read.
	 * @return current state value
	 */
	protected final int getState() {
		return state;
	}

	/**
	 * Sets the value of synchronization state.
	 * This operation has memory semantics of a {@code volatile} write.
	 * @param newState the new state value
	 */
	protected final void setState(int newState) {
		state = newState;
	}

	/**
	 * Atomically sets synchronization state to the given updated
	 * value if the current state value equals the expected value.
	 * This operation has memory semantics of a {@code volatile} read
	 * and write.
	 *
	 * @param expect the expected value
	 * @param update the new value
	 * @return {@code true} if successful. False return indicates that the actual
	 *         value was not equal to the expected value.
	 */
	protected final boolean compareAndSetState(int expect, int update) {
		// See below for intrinsics setup to support this
		return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
	}

	// Queuing utilities

	/**
	 * The number of nanoseconds for which it is faster to spin
	 * rather than to use timed park. A rough estimate suffices
	 * to improve responsiveness with very short timeouts.
	 */
	static final long spinForTimeoutThreshold = 1000L;

	/**
	 * Inserts node into queue, initializing if necessary. See picture above.
	 * @param node the node to insert
	 * @return node's predecessor
	 */
	private XaAbstractQueuedSynchronizer.Node enq(final XaAbstractQueuedSynchronizer.Node node) {
		for (;;) {
			XaAbstractQueuedSynchronizer.Node t = tail;
			if (t == null) { // Must initialize
				if (compareAndSetHead(new XaAbstractQueuedSynchronizer.Node()))
					tail = head;
			} else {
				node.prev = t;
				if (compareAndSetTail(t, node)) {
					t.next = node;
					return t;
				}
			}
		}
	}

	/**
	 * Creates and enqueues node for current thread and given mode.
	 *
	 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
	 * @return the new node
	 */
	private XaAbstractQueuedSynchronizer.Node addWaiter(XaAbstractQueuedSynchronizer.Node mode) {
		XaAbstractQueuedSynchronizer.Node node = new XaAbstractQueuedSynchronizer.Node(Thread.currentThread(), mode);
		// Try the fast path of enq; backup to full enq on failure
		XaAbstractQueuedSynchronizer.Node pred = tail;
		if (pred != null) {
			node.prev = pred;
			if (compareAndSetTail(pred, node)) {
				pred.next = node;
				return node;
			}
		}
		enq(node);
		return node;
	}

	/**
	 * Sets head of queue to be node, thus dequeuing. Called only by
	 * acquire methods.  Also nulls out unused fields for sake of GC
	 * and to suppress unnecessary signals and traversals.
	 *
	 * @param node the node
	 */
	private void setHead(XaAbstractQueuedSynchronizer.Node node) {
		head = node;
		node.thread = null;
		node.prev = null;
	}

	/**
	 * Wakes up node's successor, if one exists.
	 *
	 * @param node the node
	 */
	private void unparkSuccessor(XaAbstractQueuedSynchronizer.Node node) {
		/*
		 * If status is negative (i.e., possibly needing signal) try
		 * to clear in anticipation of signalling.  It is OK if this
		 * fails or if status is changed by waiting thread.
		 */
		int ws = node.waitStatus;
		if (ws < 0)
			compareAndSetWaitStatus(node, ws, 0);

		/*
		 * Thread to unpark is held in successor, which is normally
		 * just the next node.  But if cancelled or apparently null,
		 * traverse backwards from tail to find the actual
		 * non-cancelled successor.
		 */
		XaAbstractQueuedSynchronizer.Node s = node.next;
		if (s == null || s.waitStatus > 0) {
			s = null;
			for (XaAbstractQueuedSynchronizer.Node t = tail; t != null && t != node; t = t.prev)
				if (t.waitStatus <= 0)
					s = t;
		}
		if (s != null)
			LockSupport.unpark(s.thread);
	}

	/**
	 * Release action for shared mode -- signals successor and ensures
	 * propagation. (Note: For exclusive mode, release just amounts
	 * to calling unparkSuccessor of head if it needs signal.)
	 */
	private void doReleaseShared() {
		/*
		 * Ensure that a release propagates, even if there are other
		 * in-progress acquires/releases.  This proceeds in the usual
		 * way of trying to unparkSuccessor of head if it needs
		 * signal. But if it does not, status is set to PROPAGATE to
		 * ensure that upon release, propagation continues.
		 * Additionally, we must loop in case a new node is added
		 * while we are doing this. Also, unlike other uses of
		 * unparkSuccessor, we need to know if CAS to reset status
		 * fails, if so rechecking.
		 */
		for (;;) {
			XaAbstractQueuedSynchronizer.Node h = head;
			if (h != null && h != tail) {
				int ws = h.waitStatus;
				if (ws == XaAbstractQueuedSynchronizer.Node.SIGNAL) {
					if (!compareAndSetWaitStatus(h, XaAbstractQueuedSynchronizer.Node.SIGNAL, 0))
						continue;            // loop to recheck cases
					unparkSuccessor(h);
				}
				else if (ws == 0 &&
						!compareAndSetWaitStatus(h, 0, XaAbstractQueuedSynchronizer.Node.PROPAGATE))
					continue;                // loop on failed CAS
			}
			if (h == head)                   // loop if head changed
				break;
		}
	}

	/**
	 * Sets head of queue, and checks if successor may be waiting
	 * in shared mode, if so propagating if either propagate > 0 or
	 * PROPAGATE status was set.
	 *
	 * @param node the node
	 * @param propagate the return value from a tryAcquireShared
	 */
	private void setHeadAndPropagate(XaAbstractQueuedSynchronizer.Node node, int propagate) {
		XaAbstractQueuedSynchronizer.Node h = head; // Record old head for check below
		setHead(node);
		/*
		 * Try to signal next queued node if:
		 *   Propagation was indicated by caller,
		 *     or was recorded (as h.waitStatus either before
		 *     or after setHead) by a previous operation
		 *     (note: this uses sign-check of waitStatus because
		 *      PROPAGATE status may transition to SIGNAL.)
		 * and
		 *   The next node is waiting in shared mode,
		 *     or we don't know, because it appears null
		 *
		 * The conservatism in both of these checks may cause
		 * unnecessary wake-ups, but only when there are multiple
		 * racing acquires/releases, so most need signals now or soon
		 * anyway.
		 */
		if (propagate > 0 || h == null || h.waitStatus < 0 ||
				(h = head) == null || h.waitStatus < 0) {
			XaAbstractQueuedSynchronizer.Node s = node.next;
			if (s == null || s.isShared())
				doReleaseShared();
		}
	}

	// Utilities for various versions of acquire

	/**
	 * Cancels an ongoing attempt to acquire.
	 *
	 * @param node the node
	 */
	private void cancelAcquire(XaAbstractQueuedSynchronizer.Node node) {
		// Ignore if node doesn't exist
		if (node == null)
			return;

		node.thread = null;

		// Skip cancelled predecessors
		XaAbstractQueuedSynchronizer.Node pred = node.prev;
		while (pred.waitStatus > 0)
			node.prev = pred = pred.prev;

		// predNext is the apparent node to unsplice. CASes below will
		// fail if not, in which case, we lost race vs another cancel
		// or signal, so no further action is necessary.
		XaAbstractQueuedSynchronizer.Node predNext = pred.next;

		// Can use unconditional write instead of CAS here.
		// After this atomic step, other Nodes can skip past us.
		// Before, we are free of interference from other threads.
		node.waitStatus = XaAbstractQueuedSynchronizer.Node.CANCELLED;

		// If we are the tail, remove ourselves.
		if (node == tail && compareAndSetTail(node, pred)) {
			compareAndSetNext(pred, predNext, null);
		} else {
			// If successor needs signal, try to set pred's next-link
			// so it will get one. Otherwise wake it up to propagate.
			int ws;
			if (pred != head &&
					((ws = pred.waitStatus) == XaAbstractQueuedSynchronizer.Node.SIGNAL ||
							(ws <= 0 && compareAndSetWaitStatus(pred, ws, XaAbstractQueuedSynchronizer.Node.SIGNAL))) &&
					pred.thread != null) {
				XaAbstractQueuedSynchronizer.Node next = node.next;
				if (next != null && next.waitStatus <= 0)
					compareAndSetNext(pred, predNext, next);
			} else {
				unparkSuccessor(node);
			}

			node.next = node; // help GC
		}
	}

	/**
	 * Checks and updates status for a node that failed to acquire.
	 * Returns true if thread should block. This is the main signal
	 * control in all acquire loops.  Requires that pred == node.prev.
	 *
	 * @param pred node's predecessor holding status
	 * @param node the node
	 * @return {@code true} if thread should block
	 */
	private static boolean shouldParkAfterFailedAcquire(XaAbstractQueuedSynchronizer.Node pred, XaAbstractQueuedSynchronizer.Node node) {
		int ws = pred.waitStatus;
		if (ws == XaAbstractQueuedSynchronizer.Node.SIGNAL)
			/*
			 * This node has already set status asking a release
			 * to signal it, so it can safely park.
			 */
			return true;
		if (ws > 0) {
			/*
			 * Predecessor was cancelled. Skip over predecessors and
			 * indicate retry.
			 */
			do {
				node.prev = pred = pred.prev;
			} while (pred.waitStatus > 0);
			pred.next = node;
		} else {
			/*
			 * waitStatus must be 0 or PROPAGATE.  Indicate that we
			 * need a signal, but don't park yet.  Caller will need to
			 * retry to make sure it cannot acquire before parking.
			 */
			compareAndSetWaitStatus(pred, ws, XaAbstractQueuedSynchronizer.Node.SIGNAL);
		}
		return false;
	}

	/**
	 * Convenience method to interrupt current thread.
	 */
	static void selfInterrupt() {
		Thread.currentThread().interrupt();
	}

	/**
	 * Convenience method to park and then check if interrupted
	 *
	 * @return {@code true} if interrupted
	 */
	private final boolean parkAndCheckInterrupt() {
		LockSupport.park(this);
		return Thread.interrupted();
	}

	/*
	 * Various flavors of acquire, varying in exclusive/shared and
	 * control modes.  Each is mostly the same, but annoyingly
	 * different.  Only a little bit of factoring is possible due to
	 * interactions of exception mechanics (including ensuring that we
	 * cancel if tryAcquire throws exception) and other control, at
	 * least not without hurting performance too much.
	 */

	/**
	 * Acquires in exclusive uninterruptible mode for thread already in
	 * queue. Used by condition wait methods as well as acquire.
	 *
	 * @param node the node
	 * @param arg the acquire argument
	 * @return {@code true} if interrupted while waiting
	 */
	final boolean acquireQueued(final XaAbstractQueuedSynchronizer.Node node, int arg) {
		boolean failed = true;
		try {
			boolean interrupted = false;
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head && tryAcquire(arg)) {
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return interrupted;
				}
				if (shouldParkAfterFailedAcquire(p, node) &&
						parkAndCheckInterrupt())
					interrupted = true;
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in exclusive interruptible mode.
	 * @param arg the acquire argument
	 */
	private void doAcquireInterruptibly(int arg)
			throws InterruptedException {
		final XaAbstractQueuedSynchronizer.Node node = addWaiter(XaAbstractQueuedSynchronizer.Node.EXCLUSIVE);
		boolean failed = true;
		try {
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head && tryAcquire(arg)) {
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return;
				}
				if (shouldParkAfterFailedAcquire(p, node) &&
						parkAndCheckInterrupt())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in exclusive timed mode.
	 *
	 * @param arg the acquire argument
	 * @param nanosTimeout max wait time
	 * @return {@code true} if acquired
	 */
	private boolean doAcquireNanos(int arg, long nanosTimeout)
			throws InterruptedException {
		if (nanosTimeout <= 0L)
			return false;
		final long deadline = System.nanoTime() + nanosTimeout;
		final XaAbstractQueuedSynchronizer.Node node = addWaiter(XaAbstractQueuedSynchronizer.Node.EXCLUSIVE);
		boolean failed = true;
		try {
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head && tryAcquire(arg)) {
					setHead(node);
					p.next = null; // help GC
					failed = false;
					return true;
				}
				nanosTimeout = deadline - System.nanoTime();
				if (nanosTimeout <= 0L)
					return false;
				if (shouldParkAfterFailedAcquire(p, node) &&
						nanosTimeout > spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				if (Thread.interrupted())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared uninterruptible mode.
	 * @param arg the acquire argument
	 */
	private void doAcquireShared(int arg) {
		final XaAbstractQueuedSynchronizer.Node node = addWaiter(XaAbstractQueuedSynchronizer.Node.SHARED);
		boolean failed = true;
		try {
			boolean interrupted = false;
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);
					if (r >= 0) {
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						if (interrupted)
							selfInterrupt();
						failed = false;
						return;
					}
				}
				if (shouldParkAfterFailedAcquire(p, node) &&
						parkAndCheckInterrupt())
					interrupted = true;
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared interruptible mode.
	 * @param arg the acquire argument
	 */
	private void doAcquireSharedInterruptibly(int arg)
			throws InterruptedException {
		final XaAbstractQueuedSynchronizer.Node node = addWaiter(XaAbstractQueuedSynchronizer.Node.SHARED);
		boolean failed = true;
		try {
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);
					if (r >= 0) {
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						failed = false;
						return;
					}
				}
				if (shouldParkAfterFailedAcquire(p, node) &&
						parkAndCheckInterrupt())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	/**
	 * Acquires in shared timed mode.
	 *
	 * @param arg the acquire argument
	 * @param nanosTimeout max wait time
	 * @return {@code true} if acquired
	 */
	private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
			throws InterruptedException {
		if (nanosTimeout <= 0L)
			return false;
		final long deadline = System.nanoTime() + nanosTimeout;
		final XaAbstractQueuedSynchronizer.Node node = addWaiter(XaAbstractQueuedSynchronizer.Node.SHARED);
		boolean failed = true;
		try {
			for (;;) {
				final XaAbstractQueuedSynchronizer.Node p = node.predecessor();
				if (p == head) {
					int r = tryAcquireShared(arg);
					if (r >= 0) {
						setHeadAndPropagate(node, r);
						p.next = null; // help GC
						failed = false;
						return true;
					}
				}
				nanosTimeout = deadline - System.nanoTime();
				if (nanosTimeout <= 0L)
					return false;
				if (shouldParkAfterFailedAcquire(p, node) &&
						nanosTimeout > spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				if (Thread.interrupted())
					throw new InterruptedException();
			}
		} finally {
			if (failed)
				cancelAcquire(node);
		}
	}

	// Main exported methods

	/**
	 * Attempts to acquire in exclusive mode. This method should query
	 * if the state of the object permits it to be acquired in the
	 * exclusive mode, and if so to acquire it.
	 *
	 * <p>This method is always invoked by the thread performing
	 * acquire.  If this method reports failure, the acquire method
	 * may queue the thread, if it is not already queued, until it is
	 * signalled by a release from some other thread. This can be used
	 * to implement method {@link Lock#tryLock()}.
	 *
	 * <p>The default
	 * implementation throws {@link UnsupportedOperationException}.
	 *
	 * @param arg the acquire argument. This value is always the one
	 *        passed to an acquire method, or is the value saved on entry
	 *        to a condition wait.  The value is otherwise uninterpreted
	 *        and can represent anything you like.
	 * @return {@code true} if successful. Upon success, this object has
	 *         been acquired.
	 * @throws IllegalMonitorStateException if acquiring would place this
	 *         synchronizer in an illegal state. This exception must be
	 *         thrown in a consistent fashion for synchronization to work
	 *         correctly.
	 * @throws UnsupportedOperationException if exclusive mode is not supported
	 */
	protected boolean tryAcquire(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to set the state to reflect a release in exclusive
	 * mode.
	 *
	 * <p>This method is always invoked by the thread performing release.
	 *
	 * <p>The default implementation throws
	 * {@link UnsupportedOperationException}.
	 *
	 * @param arg the release argument. This value is always the one
	 *        passed to a release method, or the current state value upon
	 *        entry to a condition wait.  The value is otherwise
	 *        uninterpreted and can represent anything you like.
	 * @return {@code true} if this object is now in a fully released
	 *         state, so that any waiting threads may attempt to acquire;
	 *         and {@code false} otherwise.
	 * @throws IllegalMonitorStateException if releasing would place this
	 *         synchronizer in an illegal state. This exception must be
	 *         thrown in a consistent fashion for synchronization to work
	 *         correctly.
	 * @throws UnsupportedOperationException if exclusive mode is not supported
	 */
	protected boolean tryRelease(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to acquire in shared mode. This method should query if
	 * the state of the object permits it to be acquired in the shared
	 * mode, and if so to acquire it.
	 *
	 * <p>This method is always invoked by the thread performing
	 * acquire.  If this method reports failure, the acquire method
	 * may queue the thread, if it is not already queued, until it is
	 * signalled by a release from some other thread.
	 *
	 * <p>The default implementation throws {@link
	 * UnsupportedOperationException}.
	 *
	 * @param arg the acquire argument. This value is always the one
	 *        passed to an acquire method, or is the value saved on entry
	 *        to a condition wait.  The value is otherwise uninterpreted
	 *        and can represent anything you like.
	 * @return a negative value on failure; zero if acquisition in shared
	 *         mode succeeded but no subsequent shared-mode acquire can
	 *         succeed; and a positive value if acquisition in shared
	 *         mode succeeded and subsequent shared-mode acquires might
	 *         also succeed, in which case a subsequent waiting thread
	 *         must check availability. (Support for three different
	 *         return values enables this method to be used in contexts
	 *         where acquires only sometimes act exclusively.)  Upon
	 *         success, this object has been acquired.
	 * @throws IllegalMonitorStateException if acquiring would place this
	 *         synchronizer in an illegal state. This exception must be
	 *         thrown in a consistent fashion for synchronization to work
	 *         correctly.
	 * @throws UnsupportedOperationException if shared mode is not supported
	 */
	protected int tryAcquireShared(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Attempts to set the state to reflect a release in shared mode.
	 *
	 * <p>This method is always invoked by the thread performing release.
	 *
	 * <p>The default implementation throws
	 * {@link UnsupportedOperationException}.
	 *
	 * @param arg the release argument. This value is always the one
	 *        passed to a release method, or the current state value upon
	 *        entry to a condition wait.  The value is otherwise
	 *        uninterpreted and can represent anything you like.
	 * @return {@code true} if this release of shared mode may permit a
	 *         waiting acquire (shared or exclusive) to succeed; and
	 *         {@code false} otherwise
	 * @throws IllegalMonitorStateException if releasing would place this
	 *         synchronizer in an illegal state. This exception must be
	 *         thrown in a consistent fashion for synchronization to work
	 *         correctly.
	 * @throws UnsupportedOperationException if shared mode is not supported
	 */
	protected boolean tryReleaseShared(int arg) {
		throw new UnsupportedOperationException();
	}

	/**
	 * Returns {@code true} if synchronization is held exclusively with
	 * respect to the current (calling) thread.  This method is invoked
	 * upon each call to a non-waiting {@link XaAbstractQueuedSynchronizer.ConditionObject} method.
	 * (Waiting methods instead invoke {@link #release}.)
	 *
	 * <p>The default implementation throws {@link
	 * UnsupportedOperationException}. This method is invoked
	 * internally only within {@link XaAbstractQueuedSynchronizer.ConditionObject} methods, so need
	 * not be defined if conditions are not used.
	 *
	 * @return {@code true} if synchronization is held exclusively;
	 *         {@code false} otherwise
	 * @throws UnsupportedOperationException if conditions are not supported
	 */
	protected boolean isHeldExclusively() {
		throw new UnsupportedOperationException();
	}

	/**
	 * Acquires in exclusive mode, ignoring interrupts.  Implemented
	 * by invoking at least once {@link #tryAcquire},
	 * returning on success.  Otherwise the thread is queued, possibly
	 * repeatedly blocking and unblocking, invoking {@link
	 * #tryAcquire} until success.  This method can be used
	 * to implement method {@link Lock#lock}.
	 *
	 * @param arg the acquire argument.  This value is conveyed to
	 *        {@link #tryAcquire} but is otherwise uninterpreted and
	 *        can represent anything you like.
	 */
	public final void acquire(int arg) {
		if (!tryAcquire(arg) &&
				acquireQueued(addWaiter(XaAbstractQueuedSynchronizer.Node.EXCLUSIVE), arg))
			selfInterrupt();
	}

	/**
	 * Acquires in exclusive mode, aborting if interrupted.
	 * Implemented by first checking interrupt status, then invoking
	 * at least once {@link #tryAcquire}, returning on
	 * success.  Otherwise the thread is queued, possibly repeatedly
	 * blocking and unblocking, invoking {@link #tryAcquire}
	 * until success or the thread is interrupted.  This method can be
	 * used to implement method {@link Lock#lockInterruptibly}.
	 *
	 * @param arg the acquire argument.  This value is conveyed to
	 *        {@link #tryAcquire} but is otherwise uninterpreted and
	 *        can represent anything you like.
	 * @throws InterruptedException if the current thread is interrupted
	 */
	public final void acquireInterruptibly(int arg)
			throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		if (!tryAcquire(arg))
			doAcquireInterruptibly(arg);
	}

	/**
	 * Attempts to acquire in exclusive mode, aborting if interrupted,
	 * and failing if the given timeout elapses.  Implemented by first
	 * checking interrupt status, then invoking at least once {@link
	 * #tryAcquire}, returning on success.  Otherwise, the thread is
	 * queued, possibly repeatedly blocking and unblocking, invoking
	 * {@link #tryAcquire} until success or the thread is interrupted
	 * or the timeout elapses.  This method can be used to implement
	 * method {@link Lock#tryLock(long, TimeUnit)}.
	 *
	 * @param arg the acquire argument.  This value is conveyed to
	 *        {@link #tryAcquire} but is otherwise uninterpreted and
	 *        can represent anything you like.
	 * @param nanosTimeout the maximum number of nanoseconds to wait
	 * @return {@code true} if acquired; {@code false} if timed out
	 * @throws InterruptedException if the current thread is interrupted
	 */
	public final boolean tryAcquireNanos(int arg, long nanosTimeout)
			throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		return tryAcquire(arg) ||
				doAcquireNanos(arg, nanosTimeout);
	}

	/**
	 * Releases in exclusive mode.  Implemented by unblocking one or
	 * more threads if {@link #tryRelease} returns true.
	 * This method can be used to implement method {@link Lock#unlock}.
	 *
	 * @param arg the release argument.  This value is conveyed to
	 *        {@link #tryRelease} but is otherwise uninterpreted and
	 *        can represent anything you like.
	 * @return the value returned from {@link #tryRelease}
	 */
	public final boolean release(int arg) {
		if (tryRelease(arg)) {
			XaAbstractQueuedSynchronizer.Node h = head;
			if (h != null && h.waitStatus != 0)
				unparkSuccessor(h);
			return true;
		}
		return false;
	}

	/**
	 * Acquires in shared mode, ignoring interrupts.  Implemented by
	 * first invoking at least once {@link #tryAcquireShared},
	 * returning on success.  Otherwise the thread is queued, possibly
	 * repeatedly blocking and unblocking, invoking {@link
	 * #tryAcquireShared} until success.
	 *
	 * @param arg the acquire argument.  This value is conveyed to
	 *        {@link #tryAcquireShared} but is otherwise uninterpreted
	 *        and can represent anything you like.
	 */
	public final void acquireShared(int arg) {
		if (tryAcquireShared(arg) < 0)
			doAcquireShared(arg);
	}

	/**
	 * Acquires in shared mode, aborting if interrupted.  Implemented
	 * by first checking interrupt status, then invoking at least once
	 * {@link #tryAcquireShared}, returning on success.  Otherwise the
	 * thread is queued, possibly repeatedly blocking and unblocking,
	 * invoking {@link #tryAcquireShared} until success or the thread
	 * is interrupted.
	 * @param arg the acquire argument.
	 * This value is conveyed to {@link #tryAcquireShared} but is
	 * otherwise uninterpreted and can represent anything
	 * you like.
	 * @throws InterruptedException if the current thread is interrupted
	 */
	public final void acquireSharedInterruptibly(int arg)
			throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		if (tryAcquireShared(arg) < 0)
			doAcquireSharedInterruptibly(arg);
	}

	/**
	 * Attempts to acquire in shared mode, aborting if interrupted, and
	 * failing if the given timeout elapses.  Implemented by first
	 * checking interrupt status, then invoking at least once {@link
	 * #tryAcquireShared}, returning on success.  Otherwise, the
	 * thread is queued, possibly repeatedly blocking and unblocking,
	 * invoking {@link #tryAcquireShared} until success or the thread
	 * is interrupted or the timeout elapses.
	 *
	 * @param arg the acquire argument.  This value is conveyed to
	 *        {@link #tryAcquireShared} but is otherwise uninterpreted
	 *        and can represent anything you like.
	 * @param nanosTimeout the maximum number of nanoseconds to wait
	 * @return {@code true} if acquired; {@code false} if timed out
	 * @throws InterruptedException if the current thread is interrupted
	 */
	public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
			throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		return tryAcquireShared(arg) >= 0 ||
				doAcquireSharedNanos(arg, nanosTimeout);
	}

	/**
	 * Releases in shared mode.  Implemented by unblocking one or more
	 * threads if {@link #tryReleaseShared} returns true.
	 *
	 * @param arg the release argument.  This value is conveyed to
	 *        {@link #tryReleaseShared} but is otherwise uninterpreted
	 *        and can represent anything you like.
	 * @return the value returned from {@link #tryReleaseShared}
	 */
	public final boolean releaseShared(int arg) {
		if (tryReleaseShared(arg)) {
			doReleaseShared();
			return true;
		}
		return false;
	}

	// Queue inspection methods

	/**
	 * Queries whether any threads are waiting to acquire. Note that
	 * because cancellations due to interrupts and timeouts may occur
	 * at any time, a {@code true} return does not guarantee that any
	 * other thread will ever acquire.
	 *
	 * <p>In this implementation, this operation returns in
	 * constant time.
	 *
	 * @return {@code true} if there may be other threads waiting to acquire
	 */
	public final boolean hasQueuedThreads() {
		return head != tail;
	}

	/**
	 * Queries whether any threads have ever contended to acquire this
	 * synchronizer; that is if an acquire method has ever blocked.
	 *
	 * <p>In this implementation, this operation returns in
	 * constant time.
	 *
	 * @return {@code true} if there has ever been contention
	 */
	public final boolean hasContended() {
		return head != null;
	}

	/**
	 * Returns the first (longest-waiting) thread in the queue, or
	 * {@code null} if no threads are currently queued.
	 *
	 * <p>In this implementation, this operation normally returns in
	 * constant time, but may iterate upon contention if other threads are
	 * concurrently modifying the queue.
	 *
	 * @return the first (longest-waiting) thread in the queue, or
	 *         {@code null} if no threads are currently queued
	 */
	public final Thread getFirstQueuedThread() {
		// handle only fast path, else relay
		return (head == tail) ? null : fullGetFirstQueuedThread();
	}

	/**
	 * Version of getFirstQueuedThread called when fastpath fails
	 */
	private Thread fullGetFirstQueuedThread() {
		/*
		 * The first node is normally head.next. Try to get its
		 * thread field, ensuring consistent reads: If thread
		 * field is nulled out or s.prev is no longer head, then
		 * some other thread(s) concurrently performed setHead in
		 * between some of our reads. We try this twice before
		 * resorting to traversal.
		 */
		XaAbstractQueuedSynchronizer.Node h, s;
		Thread st;
		if (((h = head) != null && (s = h.next) != null &&
				s.prev == head && (st = s.thread) != null) ||
				((h = head) != null && (s = h.next) != null &&
						s.prev == head && (st = s.thread) != null))
			return st;

		/*
		 * Head's next field might not have been set yet, or may have
		 * been unset after setHead. So we must check to see if tail
		 * is actually first node. If not, we continue on, safely
		 * traversing from tail back to head to find first,
		 * guaranteeing termination.
		 */

		XaAbstractQueuedSynchronizer.Node t = tail;
		Thread firstThread = null;
		while (t != null && t != head) {
			Thread tt = t.thread;
			if (tt != null)
				firstThread = tt;
			t = t.prev;
		}
		return firstThread;
	}

	/**
	 * Returns true if the given thread is currently queued.
	 *
	 * <p>This implementation traverses the queue to determine
	 * presence of the given thread.
	 *
	 * @param thread the thread
	 * @return {@code true} if the given thread is on the queue
	 * @throws NullPointerException if the thread is null
	 */
	public final boolean isQueued(Thread thread) {
		if (thread == null)
			throw new NullPointerException();
		for (XaAbstractQueuedSynchronizer.Node p = tail; p != null; p = p.prev)
			if (p.thread == thread)
				return true;
		return false;
	}

	/**
	 * Returns {@code true} if the apparent first queued thread, if one
	 * exists, is waiting in exclusive mode.  If this method returns
	 * {@code true}, and the current thread is attempting to acquire in
	 * shared mode (that is, this method is invoked from {@link
	 * #tryAcquireShared}) then it is guaranteed that the current thread
	 * is not the first queued thread.  Used only as a heuristic in
	 * ReentrantReadWriteLock.
	 */
	final boolean apparentlyFirstQueuedIsExclusive() {
		XaAbstractQueuedSynchronizer.Node h, s;
		return (h = head) != null &&
				(s = h.next)  != null &&
				!s.isShared()         &&
				s.thread != null;
	}

	/**
	 * Queries whether any threads have been waiting to acquire longer
	 * than the current thread.
	 *
	 * <p>An invocation of this method is equivalent to (but may be
	 * more efficient than):
	 *  <pre> {@code
	 * getFirstQueuedThread() != Thread.currentThread() &&
	 * hasQueuedThreads()}</pre>
	 *
	 * <p>Note that because cancellations due to interrupts and
	 * timeouts may occur at any time, a {@code true} return does not
	 * guarantee that some other thread will acquire before the current
	 * thread.  Likewise, it is possible for another thread to win a
	 * race to enqueue after this method has returned {@code false},
	 * due to the queue being empty.
	 *
	 * <p>This method is designed to be used by a fair synchronizer to
	 * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
	 * Such a synchronizer's {@link #tryAcquire} method should return
	 * {@code false}, and its {@link #tryAcquireShared} method should
	 * return a negative value, if this method returns {@code true}
	 * (unless this is a reentrant acquire).  For example, the {@code
	 * tryAcquire} method for a fair, reentrant, exclusive mode
	 * synchronizer might look like this:
	 *
	 *  <pre> {@code
	 * protected boolean tryAcquire(int arg) {
	 *   if (isHeldExclusively()) {
	 *     // A reentrant acquire; increment hold count
	 *     return true;
	 *   } else if (hasQueuedPredecessors()) {
	 *     return false;
	 *   } else {
	 *     // try to acquire normally
	 *   }
	 * }}</pre>
	 *
	 * @return {@code true} if there is a queued thread preceding the
	 *         current thread, and {@code false} if the current thread
	 *         is at the head of the queue or the queue is empty
	 * @since 1.7
	 */
	public final boolean hasQueuedPredecessors() {
		// The correctness of this depends on head being initialized
		// before tail and on head.next being accurate if the current
		// thread is first in queue.
		XaAbstractQueuedSynchronizer.Node t = tail; // Read fields in reverse initialization order
		XaAbstractQueuedSynchronizer.Node h = head;
		XaAbstractQueuedSynchronizer.Node s;
		return h != t &&
				((s = h.next) == null || s.thread != Thread.currentThread());
	}


	// Instrumentation and monitoring methods

	/**
	 * Returns an estimate of the number of threads waiting to
	 * acquire.  The value is only an estimate because the number of
	 * threads may change dynamically while this method traverses
	 * internal data structures.  This method is designed for use in
	 * monitoring system state, not for synchronization
	 * control.
	 *
	 * @return the estimated number of threads waiting to acquire
	 */
	public final int getQueueLength() {
		int n = 0;
		for (XaAbstractQueuedSynchronizer.Node p = tail; p != null; p = p.prev) {
			if (p.thread != null)
				++n;
		}
		return n;
	}

	/**
	 * Returns a collection containing threads that may be waiting to
	 * acquire.  Because the actual set of threads may change
	 * dynamically while constructing this result, the returned
	 * collection is only a best-effort estimate.  The elements of the
	 * returned collection are in no particular order.  This method is
	 * designed to facilitate construction of subclasses that provide
	 * more extensive monitoring facilities.
	 *
	 * @return the collection of threads
	 */
	public final Collection<Thread> getQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (XaAbstractQueuedSynchronizer.Node p = tail; p != null; p = p.prev) {
			Thread t = p.thread;
			if (t != null)
				list.add(t);
		}
		return list;
	}

	/**
	 * Returns a collection containing threads that may be waiting to
	 * acquire in exclusive mode. This has the same properties
	 * as {@link #getQueuedThreads} except that it only returns
	 * those threads waiting due to an exclusive acquire.
	 *
	 * @return the collection of threads
	 */
	public final Collection<Thread> getExclusiveQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (XaAbstractQueuedSynchronizer.Node p = tail; p != null; p = p.prev) {
			if (!p.isShared()) {
				Thread t = p.thread;
				if (t != null)
					list.add(t);
			}
		}
		return list;
	}

	/**
	 * Returns a collection containing threads that may be waiting to
	 * acquire in shared mode. This has the same properties
	 * as {@link #getQueuedThreads} except that it only returns
	 * those threads waiting due to a shared acquire.
	 *
	 * @return the collection of threads
	 */
	public final Collection<Thread> getSharedQueuedThreads() {
		ArrayList<Thread> list = new ArrayList<Thread>();
		for (XaAbstractQueuedSynchronizer.Node p = tail; p != null; p = p.prev) {
			if (p.isShared()) {
				Thread t = p.thread;
				if (t != null)
					list.add(t);
			}
		}
		return list;
	}

	/**
	 * Returns a string identifying this synchronizer, as well as its state.
	 * The state, in brackets, includes the String {@code "State ="}
	 * followed by the current value of {@link #getState}, and either
	 * {@code "nonempty"} or {@code "empty"} depending on whether the
	 * queue is empty.
	 *
	 * @return a string identifying this synchronizer, as well as its state
	 */
	public String toString() {
		int s = getState();
		String q  = hasQueuedThreads() ? "non" : "";
		return super.toString() +
				"[State = " + s + ", " + q + "empty queue]";
	}


	// Internal support methods for Conditions

	/**
	 * Returns true if a node, always one that was initially placed on
	 * a condition queue, is now waiting to reacquire on sync queue.
	 * @param node the node
	 * @return true if is reacquiring
	 */
	final boolean isOnSyncQueue(XaAbstractQueuedSynchronizer.Node node) {
		if (node.waitStatus == XaAbstractQueuedSynchronizer.Node.CONDITION || node.prev == null)
			return false;
		if (node.next != null) // If has successor, it must be on queue
			return true;
		/*
		 * node.prev can be non-null, but not yet on queue because
		 * the CAS to place it on queue can fail. So we have to
		 * traverse from tail to make sure it actually made it.  It
		 * will always be near the tail in calls to this method, and
		 * unless the CAS failed (which is unlikely), it will be
		 * there, so we hardly ever traverse much.
		 */
		return findNodeFromTail(node);
	}

	/**
	 * Returns true if node is on sync queue by searching backwards from tail.
	 * Called only when needed by isOnSyncQueue.
	 * @return true if present
	 */
	private boolean findNodeFromTail(XaAbstractQueuedSynchronizer.Node node) {
		XaAbstractQueuedSynchronizer.Node t = tail;
		for (;;) {
			if (t == node)
				return true;
			if (t == null)
				return false;
			t = t.prev;
		}
	}

	/**
	 * Transfers a node from a condition queue onto sync queue.
	 * Returns true if successful.
	 * @param node the node
	 * @return true if successfully transferred (else the node was
	 * cancelled before signal)
	 */
	final boolean transferForSignal(XaAbstractQueuedSynchronizer.Node node) {
		/*
		 * If cannot change waitStatus, the node has been cancelled.
		 */
		if (!compareAndSetWaitStatus(node, XaAbstractQueuedSynchronizer.Node.CONDITION, 0))
			return false;

		/*
		 * Splice onto queue and try to set waitStatus of predecessor to
		 * indicate that thread is (probably) waiting. If cancelled or
		 * attempt to set waitStatus fails, wake up to resync (in which
		 * case the waitStatus can be transiently and harmlessly wrong).
		 */
		XaAbstractQueuedSynchronizer.Node p = enq(node);
		int ws = p.waitStatus;
		if (ws > 0 || !compareAndSetWaitStatus(p, ws, XaAbstractQueuedSynchronizer.Node.SIGNAL))
			LockSupport.unpark(node.thread);
		return true;
	}

	/**
	 * Transfers node, if necessary, to sync queue after a cancelled wait.
	 * Returns true if thread was cancelled before being signalled.
	 *
	 * @param node the node
	 * @return true if cancelled before the node was signalled
	 */
	final boolean transferAfterCancelledWait(XaAbstractQueuedSynchronizer.Node node) {
		if (compareAndSetWaitStatus(node, XaAbstractQueuedSynchronizer.Node.CONDITION, 0)) {
			enq(node);
			return true;
		}
		/*
		 * If we lost out to a signal(), then we can't proceed
		 * until it finishes its enq().  Cancelling during an
		 * incomplete transfer is both rare and transient, so just
		 * spin.
		 */
		while (!isOnSyncQueue(node))
			Thread.yield();
		return false;
	}

	/**
	 * Invokes release with current state value; returns saved state.
	 * Cancels node and throws exception on failure.
	 * @param node the condition node for this wait
	 * @return previous sync state
	 */
	final int fullyRelease(XaAbstractQueuedSynchronizer.Node node) {
		boolean failed = true;
		try {
			int savedState = getState();
			if (release(savedState)) {
				failed = false;
				return savedState;
			} else {
				throw new IllegalMonitorStateException();
			}
		} finally {
			if (failed)
				node.waitStatus = XaAbstractQueuedSynchronizer.Node.CANCELLED;
		}
	}

	// Instrumentation methods for conditions

	/**
	 * Queries whether the given ConditionObject
	 * uses this synchronizer as its lock.
	 *
	 * @param condition the condition
	 * @return {@code true} if owned
	 * @throws NullPointerException if the condition is null
	 */
	public final boolean owns(XaAbstractQueuedSynchronizer.ConditionObject condition) {
		return condition.isOwnedBy(this);
	}

	/**
	 * Queries whether any threads are waiting on the given condition
	 * associated with this synchronizer. Note that because timeouts
	 * and interrupts may occur at any time, a {@code true} return
	 * does not guarantee that a future {@code signal} will awaken
	 * any threads.  This method is designed primarily for use in
	 * monitoring of the system state.
	 *
	 * @param condition the condition
	 * @return {@code true} if there are any waiting threads
	 * @throws IllegalMonitorStateException if exclusive synchronization
	 *         is not held
	 * @throws IllegalArgumentException if the given condition is
	 *         not associated with this synchronizer
	 * @throws NullPointerException if the condition is null
	 */
	public final boolean hasWaiters(XaAbstractQueuedSynchronizer.ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.hasWaiters();
	}

	/**
	 * Returns an estimate of the number of threads waiting on the
	 * given condition associated with this synchronizer. Note that
	 * because timeouts and interrupts may occur at any time, the
	 * estimate serves only as an upper bound on the actual number of
	 * waiters.  This method is designed for use in monitoring of the
	 * system state, not for synchronization control.
	 *
	 * @param condition the condition
	 * @return the estimated number of waiting threads
	 * @throws IllegalMonitorStateException if exclusive synchronization
	 *         is not held
	 * @throws IllegalArgumentException if the given condition is
	 *         not associated with this synchronizer
	 * @throws NullPointerException if the condition is null
	 */
	public final int getWaitQueueLength(XaAbstractQueuedSynchronizer.ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.getWaitQueueLength();
	}

	/**
	 * Returns a collection containing those threads that may be
	 * waiting on the given condition associated with this
	 * synchronizer.  Because the actual set of threads may change
	 * dynamically while constructing this result, the returned
	 * collection is only a best-effort estimate. The elements of the
	 * returned collection are in no particular order.
	 *
	 * @param condition the condition
	 * @return the collection of threads
	 * @throws IllegalMonitorStateException if exclusive synchronization
	 *         is not held
	 * @throws IllegalArgumentException if the given condition is
	 *         not associated with this synchronizer
	 * @throws NullPointerException if the condition is null
	 */
	public final Collection<Thread> getWaitingThreads(XaAbstractQueuedSynchronizer.ConditionObject condition) {
		if (!owns(condition))
			throw new IllegalArgumentException("Not owner");
		return condition.getWaitingThreads();
	}

	/**
	 * Condition implementation for a {@link
	 * XaAbstractQueuedSynchronizer} serving as the basis of a {@link
	 * Lock} implementation.
	 *
	 * <p>Method documentation for this class describes mechanics,
	 * not behavioral specifications from the point of view of Lock
	 * and Condition users. Exported versions of this class will in
	 * general need to be accompanied by documentation describing
	 * condition semantics that rely on those of the associated
	 * {@code AbstractQueuedSynchronizer}.
	 *
	 * <p>This class is Serializable, but all fields are transient,
	 * so deserialized conditions have no waiters.
	 */
	public class ConditionObject implements Condition, java.io.Serializable {
		private static final long serialVersionUID = 1173984872572414699L;
		/** First node of condition queue. */
		private transient XaAbstractQueuedSynchronizer.Node firstWaiter;
		/** Last node of condition queue. */
		private transient XaAbstractQueuedSynchronizer.Node lastWaiter;

		/**
		 * Creates a new {@code ConditionObject} instance.
		 */
		public ConditionObject() { }

		// Internal methods

		/**
		 * Adds a new waiter to wait queue.
		 * @return its new wait node
		 */
		private XaAbstractQueuedSynchronizer.Node addConditionWaiter() {
			XaAbstractQueuedSynchronizer.Node t = lastWaiter;
			// If lastWaiter is cancelled, clean out.
			if (t != null && t.waitStatus != XaAbstractQueuedSynchronizer.Node.CONDITION) {
				unlinkCancelledWaiters();
				t = lastWaiter;
			}
			XaAbstractQueuedSynchronizer.Node node = new XaAbstractQueuedSynchronizer.Node(Thread.currentThread(), XaAbstractQueuedSynchronizer.Node.CONDITION);
			if (t == null)
				firstWaiter = node;
			else
				t.nextWaiter = node;
			lastWaiter = node;
			return node;
		}

		/**
		 * Removes and transfers nodes until hit non-cancelled one or
		 * null. Split out from signal in part to encourage compilers
		 * to inline the case of no waiters.
		 * @param first (non-null) the first node on condition queue
		 */
		private void doSignal(XaAbstractQueuedSynchronizer.Node first) {
			do {
				if ( (firstWaiter = first.nextWaiter) == null)
					lastWaiter = null;
				first.nextWaiter = null;
			} while (!transferForSignal(first) &&
					(first = firstWaiter) != null);
		}

		/**
		 * Removes and transfers all nodes.
		 * @param first (non-null) the first node on condition queue
		 */
		private void doSignalAll(XaAbstractQueuedSynchronizer.Node first) {
			lastWaiter = firstWaiter = null;
			do {
				XaAbstractQueuedSynchronizer.Node next = first.nextWaiter;
				first.nextWaiter = null;
				transferForSignal(first);
				first = next;
			} while (first != null);
		}

		/**
		 * Unlinks cancelled waiter nodes from condition queue.
		 * Called only while holding lock. This is called when
		 * cancellation occurred during condition wait, and upon
		 * insertion of a new waiter when lastWaiter is seen to have
		 * been cancelled. This method is needed to avoid garbage
		 * retention in the absence of signals. So even though it may
		 * require a full traversal, it comes into play only when
		 * timeouts or cancellations occur in the absence of
		 * signals. It traverses all nodes rather than stopping at a
		 * particular target to unlink all pointers to garbage nodes
		 * without requiring many re-traversals during cancellation
		 * storms.
		 */
		private void unlinkCancelledWaiters() {
			XaAbstractQueuedSynchronizer.Node t = firstWaiter;
			XaAbstractQueuedSynchronizer.Node trail = null;
			while (t != null) {
				XaAbstractQueuedSynchronizer.Node next = t.nextWaiter;
				if (t.waitStatus != XaAbstractQueuedSynchronizer.Node.CONDITION) {
					t.nextWaiter = null;
					if (trail == null)
						firstWaiter = next;
					else
						trail.nextWaiter = next;
					if (next == null)
						lastWaiter = trail;
				}
				else
					trail = t;
				t = next;
			}
		}

		// public methods

		/**
		 * Moves the longest-waiting thread, if one exists, from the
		 * wait queue for this condition to the wait queue for the
		 * owning lock.
		 *
		 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
		 *         returns {@code false}
		 */
		public final void signal() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			XaAbstractQueuedSynchronizer.Node first = firstWaiter;
			if (first != null)
				doSignal(first);
		}

		/**
		 * Moves all threads from the wait queue for this condition to
		 * the wait queue for the owning lock.
		 *
		 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
		 *         returns {@code false}
		 */
		public final void signalAll() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			XaAbstractQueuedSynchronizer.Node first = firstWaiter;
			if (first != null)
				doSignalAll(first);
		}

		/**
		 * Implements uninterruptible condition wait.
		 * <ol>
		 * <li> Save lock state returned by {@link #getState}.
		 * <li> Invoke {@link #release} with saved state as argument,
		 *      throwing IllegalMonitorStateException if it fails.
		 * <li> Block until signalled.
		 * <li> Reacquire by invoking specialized version of
		 *      {@link #acquire} with saved state as argument.
		 * </ol>
		 */
		public final void awaitUninterruptibly() {
			XaAbstractQueuedSynchronizer.Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			boolean interrupted = false;
			while (!isOnSyncQueue(node)) {
				LockSupport.park(this);
				if (Thread.interrupted())
					interrupted = true;
			}
			if (acquireQueued(node, savedState) || interrupted)
				selfInterrupt();
		}

		/*
		 * For interruptible waits, we need to track whether to throw
		 * InterruptedException, if interrupted while blocked on
		 * condition, versus reinterrupt current thread, if
		 * interrupted while blocked waiting to re-acquire.
		 */

		/** Mode meaning to reinterrupt on exit from wait */
		private static final int REINTERRUPT =  1;
		/** Mode meaning to throw InterruptedException on exit from wait */
		private static final int THROW_IE    = -1;

		/**
		 * Checks for interrupt, returning THROW_IE if interrupted
		 * before signalled, REINTERRUPT if after signalled, or
		 * 0 if not interrupted.
		 */
		private int checkInterruptWhileWaiting(XaAbstractQueuedSynchronizer.Node node) {
			return Thread.interrupted() ?
					(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
					0;
		}

		/**
		 * Throws InterruptedException, reinterrupts current thread, or
		 * does nothing, depending on mode.
		 */
		private void reportInterruptAfterWait(int interruptMode)
				throws InterruptedException {
			if (interruptMode == THROW_IE)
				throw new InterruptedException();
			else if (interruptMode == REINTERRUPT)
				selfInterrupt();
		}

		/**
		 * Implements interruptible condition wait.
		 * <ol>
		 * <li> If current thread is interrupted, throw InterruptedException.
		 * <li> Save lock state returned by {@link #getState}.
		 * <li> Invoke {@link #release} with saved state as argument,
		 *      throwing IllegalMonitorStateException if it fails.
		 * <li> Block until signalled or interrupted.
		 * <li> Reacquire by invoking specialized version of
		 *      {@link #acquire} with saved state as argument.
		 * <li> If interrupted while blocked in step 4, throw InterruptedException.
		 * </ol>
		 */
		public final void await() throws InterruptedException {
			if (Thread.interrupted())
				throw new InterruptedException();
			XaAbstractQueuedSynchronizer.Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				LockSupport.park(this);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null) // clean up if cancelled
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
		}

		/**
		 * Implements timed condition wait.
		 * <ol>
		 * <li> If current thread is interrupted, throw InterruptedException.
		 * <li> Save lock state returned by {@link #getState}.
		 * <li> Invoke {@link #release} with saved state as argument,
		 *      throwing IllegalMonitorStateException if it fails.
		 * <li> Block until signalled, interrupted, or timed out.
		 * <li> Reacquire by invoking specialized version of
		 *      {@link #acquire} with saved state as argument.
		 * <li> If interrupted while blocked in step 4, throw InterruptedException.
		 * </ol>
		 */
		public final long awaitNanos(long nanosTimeout)
				throws InterruptedException {
			if (Thread.interrupted())
				throw new InterruptedException();
			XaAbstractQueuedSynchronizer.Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			final long deadline = System.nanoTime() + nanosTimeout;
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (nanosTimeout <= 0L) {
					transferAfterCancelledWait(node);
					break;
				}
				if (nanosTimeout >= spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
				nanosTimeout = deadline - System.nanoTime();
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return deadline - System.nanoTime();
		}

		/**
		 * Implements absolute timed condition wait.
		 * <ol>
		 * <li> If current thread is interrupted, throw InterruptedException.
		 * <li> Save lock state returned by {@link #getState}.
		 * <li> Invoke {@link #release} with saved state as argument,
		 *      throwing IllegalMonitorStateException if it fails.
		 * <li> Block until signalled, interrupted, or timed out.
		 * <li> Reacquire by invoking specialized version of
		 *      {@link #acquire} with saved state as argument.
		 * <li> If interrupted while blocked in step 4, throw InterruptedException.
		 * <li> If timed out while blocked in step 4, return false, else true.
		 * </ol>
		 */
		public final boolean awaitUntil(Date deadline)
				throws InterruptedException {
			long abstime = deadline.getTime();
			if (Thread.interrupted())
				throw new InterruptedException();
			XaAbstractQueuedSynchronizer.Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			boolean timedout = false;
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (System.currentTimeMillis() > abstime) {
					timedout = transferAfterCancelledWait(node);
					break;
				}
				LockSupport.parkUntil(this, abstime);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return !timedout;
		}

		/**
		 * Implements timed condition wait.
		 * <ol>
		 * <li> If current thread is interrupted, throw InterruptedException.
		 * <li> Save lock state returned by {@link #getState}.
		 * <li> Invoke {@link #release} with saved state as argument,
		 *      throwing IllegalMonitorStateException if it fails.
		 * <li> Block until signalled, interrupted, or timed out.
		 * <li> Reacquire by invoking specialized version of
		 *      {@link #acquire} with saved state as argument.
		 * <li> If interrupted while blocked in step 4, throw InterruptedException.
		 * <li> If timed out while blocked in step 4, return false, else true.
		 * </ol>
		 */
		public final boolean await(long time, TimeUnit unit)
				throws InterruptedException {
			long nanosTimeout = unit.toNanos(time);
			if (Thread.interrupted())
				throw new InterruptedException();
			XaAbstractQueuedSynchronizer.Node node = addConditionWaiter();
			int savedState = fullyRelease(node);
			final long deadline = System.nanoTime() + nanosTimeout;
			boolean timedout = false;
			int interruptMode = 0;
			while (!isOnSyncQueue(node)) {
				if (nanosTimeout <= 0L) {
					timedout = transferAfterCancelledWait(node);
					break;
				}
				if (nanosTimeout >= spinForTimeoutThreshold)
					LockSupport.parkNanos(this, nanosTimeout);
				if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
					break;
				nanosTimeout = deadline - System.nanoTime();
			}
			if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
				interruptMode = REINTERRUPT;
			if (node.nextWaiter != null)
				unlinkCancelledWaiters();
			if (interruptMode != 0)
				reportInterruptAfterWait(interruptMode);
			return !timedout;
		}

		//  support for instrumentation

		/**
		 * Returns true if this condition was created by the given
		 * synchronization object.
		 *
		 * @return {@code true} if owned
		 */
		final boolean isOwnedBy(XaAbstractQueuedSynchronizer sync) {
			return sync == XaAbstractQueuedSynchronizer.this;
		}

		/**
		 * Queries whether any threads are waiting on this condition.
		 * Implements {@link XaAbstractQueuedSynchronizer#hasWaiters(XaAbstractQueuedSynchronizer.ConditionObject)}.
		 *
		 * @return {@code true} if there are any waiting threads
		 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
		 *         returns {@code false}
		 */
		protected final boolean hasWaiters() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			for (XaAbstractQueuedSynchronizer.Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == XaAbstractQueuedSynchronizer.Node.CONDITION)
					return true;
			}
			return false;
		}

		/**
		 * Returns an estimate of the number of threads waiting on
		 * this condition.
		 * Implements {@link XaAbstractQueuedSynchronizer#getWaitQueueLength(XaAbstractQueuedSynchronizer.ConditionObject)}.
		 *
		 * @return the estimated number of waiting threads
		 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
		 *         returns {@code false}
		 */
		protected final int getWaitQueueLength() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			int n = 0;
			for (XaAbstractQueuedSynchronizer.Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == XaAbstractQueuedSynchronizer.Node.CONDITION)
					++n;
			}
			return n;
		}

		/**
		 * Returns a collection containing those threads that may be
		 * waiting on this Condition.
		 * Implements {@link XaAbstractQueuedSynchronizer#getWaitingThreads(XaAbstractQueuedSynchronizer.ConditionObject)}.
		 *
		 * @return the collection of threads
		 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
		 *         returns {@code false}
		 */
		protected final Collection<Thread> getWaitingThreads() {
			if (!isHeldExclusively())
				throw new IllegalMonitorStateException();
			ArrayList<Thread> list = new ArrayList<Thread>();
			for (XaAbstractQueuedSynchronizer.Node w = firstWaiter; w != null; w = w.nextWaiter) {
				if (w.waitStatus == XaAbstractQueuedSynchronizer.Node.CONDITION) {
					Thread t = w.thread;
					if (t != null)
						list.add(t);
				}
			}
			return list;
		}
	}

	/**
	 * Setup to support compareAndSet. We need to natively implement
	 * this here: For the sake of permitting future enhancements, we
	 * cannot explicitly subclass AtomicInteger, which would be
	 * efficient and useful otherwise. So, as the lesser of evils, we
	 * natively implement using hotspot intrinsics API. And while we
	 * are at it, we do the same for other CASable fields (which could
	 * otherwise be done with atomic field updaters).
	 */
	private static final Unsafe unsafe = Unsafe.getUnsafe();
	private static final long stateOffset;
	private static final long headOffset;
	private static final long tailOffset;
	private static final long waitStatusOffset;
	private static final long nextOffset;

	static {
		try {
			stateOffset = unsafe.objectFieldOffset
					(XaAbstractQueuedSynchronizer.class.getDeclaredField("state"));
			headOffset = unsafe.objectFieldOffset
					(XaAbstractQueuedSynchronizer.class.getDeclaredField("head"));
			tailOffset = unsafe.objectFieldOffset
					(XaAbstractQueuedSynchronizer.class.getDeclaredField("tail"));
			waitStatusOffset = unsafe.objectFieldOffset
					(XaAbstractQueuedSynchronizer.Node.class.getDeclaredField("waitStatus"));
			nextOffset = unsafe.objectFieldOffset
					(XaAbstractQueuedSynchronizer.Node.class.getDeclaredField("next"));

		} catch (Exception ex) { throw new Error(ex); }
	}

	/**
	 * CAS head field. Used only by enq.
	 */
	private final boolean compareAndSetHead(XaAbstractQueuedSynchronizer.Node update) {
		return unsafe.compareAndSwapObject(this, headOffset, null, update);
	}

	/**
	 * CAS tail field. Used only by enq.
	 */
	private final boolean compareAndSetTail(XaAbstractQueuedSynchronizer.Node expect, XaAbstractQueuedSynchronizer.Node update) {
		return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
	}

	/**
	 * CAS waitStatus field of a node.
	 */
	private static final boolean compareAndSetWaitStatus(XaAbstractQueuedSynchronizer.Node node,
	                                                     int expect,
	                                                     int update) {
		return unsafe.compareAndSwapInt(node, waitStatusOffset,
				expect, update);
	}

	/**
	 * CAS next field of a node.
	 */
	private static final boolean compareAndSetNext(XaAbstractQueuedSynchronizer.Node node,
	                                               XaAbstractQueuedSynchronizer.Node expect,
	                                               XaAbstractQueuedSynchronizer.Node update) {
		return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
	}
}
